This inpatient study includes a detoxification and two 18-day study phases. Upon study initiation, participants are tapered with morphine before study phase 1 starts, when they are randomized to receive placebo or 50 mg MN-166 BID (po at 0800 and 2000 hr), and then switched and stabilized on the medication. Thereafter, participants will complete 6 laboratory sessions over 9-10 days.